fix timezone caching bug, fixes CNVS-7204

make sure we use the timezone-specific date in the cache key (Date.today
isn't timezone aware)

test plan:
1. make sure you have redis enabled
2. set your canvas timezone to something other than UTC
3. create an assignment that is due tomorrow at midnight (in said
   timezone)
4. view your dashboard sometime in the window where your "today" != UTC
   "today". for example, if you are on MDT, that would be sometime between
   6pm and midnight
5. Coming Up should list it as "Tomorrow"
6. check it again tomorrow *before* that window (e.g. before 6pm MDT)
7. Coming Up should list it as "Today" (prior to this fix it would list
   it incorrectly as "Tomorrow" from 12am-6pm)

Change-Id: Ib3c9e74388741ad5f74ebd9ead3f07dd63552dff
Reviewed-on: https://gerrit.instructure.com/22731
Reviewed-by: Jon Jensen <jon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Marc LeGendre <marc@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>

Author: jenseng

app/views/courses/_recent_event.html.erb

@@ -18,7 +18,7 @@
           submission || 'no_submission',
           recent_event || 'blank_event',
           (recent_event.due_at if is_assignment),
-          Date.today,
+          Time.zone.today,
           Time.zone.name,
           recent_event.cached_context_grants_right?(@current_user, nil, :manage_grades),
           recent_event.cached_context_grants_right?(@current_user, nil, :participate_as_student)